1. Field of the Invention
This invention relates to a voltage-dependent nonlinear resistor, more particularly to a voltage-dependent nonlinear resistor that is based on zinc oxide (ZnO) that is suitable for use as a device for protection from overvoltage.
2. Discussion of the Related Art
Varistors and arresters based on silicon carbide (SiC), selenium (Se), silicon (Si) or zinc oxide (ZnO) are conventionally used to protect electronic and electric devices from overvoltage. Among them, those varistors and arresters which are based on ZnO have advantages such as low residual voltage and a high voltage-dependent nonlinearity index. Thus, they are suitable for protecting those apparatus from overvoltage, such as semiconductor devices, that have a small overvoltage withstand capability. Under these circumstances, ZnO based varistors and arresters have increasingly supplanted SiC based versions. They are also used extensively as arresters for the purpose of protecting electric power apparatus.
Examined Japanese Patent Publication No. 25205/1989 describes a voltage-dependent nonlinear resistor that is produced by sintering a composition that contains ZnO as a chief component and the following as auxiliary components: 0.08-5.0 atomic percent(atm%) of a rare earth element; 0.1-10.0 atm% of cobalt (Co); 0.01-5.0 atm% at least one of magnesium (Mg) and calcium (Ca); 0.01-1.0 atm% of at least one of potassium (K), cesium (Cs) and rubidium (Rb); 0.01-1.0 atm% of chromium (Cr); 5.times.10.sup.-4 to 1.times.10.sup.-1 atm% of boron (B); and 1.times.10.sup.-4 to 5.times.10.sup.-2 atm% of at least one of aluminum (Al), gallium (Ga) and indium (In). The resulting resistor has a high voltage-dependent nonlinearity, a great surge withstand capability and a good life characteristics under voltage application.
Examined Japanese Patent Publication No. 14924/1987 describes a voltage-dependent nonlinear resistor that is produced by sintering a composition that contains ZnO as a chief component and the following as auxiliary components: 0.1-5.0 atm% of praseodymium (Pr); 0.5-5.0 atm% of cobalt (Co); 0.06-0.6 atm% in total of at least two of potassium (K), cesium (Cs) and rubidium (Rb); and 0.05-0.5 atm% of chromium (Cr). This resistor has a good voltage-dependent nonlinearity.
However, the two voltage-dependent nonlinear resistors described above still have the following problems. The performance of voltage-dependent nonlinear resistors is evaluated by two characteristics, the voltage per unit thickness with a current flowing at 1 mA [which is hereunder expressed as V.sub.1 mA /t (V/mm)] and the surge energy absorbing capability per unit volume [which is hereunder referred to as "permissible energy" (kJ/cm.sup.3)]. Since the permissible energy decreases with increasing V.sub.1 mA /t, the value of V.sub.1 mA /t is conventionally controlled not to exceed about 200 V/mm by adjusting the sintering conditions or other factors to suppress the growth of crystal grains. However, in certain applications such as where ZnO based resistors are used in surge arresters, not only the dimensions such as height but also the production cost have to be reduced. To meet this need, it is desired to increase the value of V.sub.1 mA /t to at least 300 V/mm.